The thermal and mass spectral fragmentation of N-butyl thiolo-, thiono-, and dithio-chloroformate

Article abstract of DOI:10.1080/10426507.2011.582801

Thermal decomposition of n-butyl thiolochloroformate at 150C follows a similar pattern to that reported previously for n-butyl chloroformate, to give butyl chloride which is largely rearranged to the s-butyl isomer. An ion-pair mechanism, involving 1,2-and 1,3-hydride shifts, is proposed. The less stable thiono-and dithio-chloroformates decompose to give lower yields of butyl chlorides (mainly without rearrangement) together with numerous other byproducts, indicating the operation of a more complex combination of reaction pathways. The mass spectra of all three thio compounds exhibit molecular ions; the most prominent fragment ions in their spectra are the n-butyl cation, and the radical cations COS+ and CS₂⁺. Numerous chlorine-containing ions of low intensity are also observed, and their mode of formation is discussed. Copyright Taylor &Francis Group, LLC.

Full text of DOI:10.1080/10426507.2011.582801

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Phosphorus, Sulfur, and Silicon and the
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The Thermal and Mass Spectral
Fragmentation of N-Butyl Thiolo-,
Thiono-, and Dithio-Chloroformate
Harry R. Hudson a & Andrew J. Koplick a
a London Metropolitan University, London, UK
Version of record first published: 11 Aug 2011.
To cite this article: Harry R. Hudson & Andrew J. Koplick (2011): The Thermal and Mass Spectral
Fragmentation of N-Butyl Thiolo-, Thiono-, and Dithio-Chloroformate, Phosphorus, Sulfur, and Silicon
and the Related Elements, 186:8, 1635-1649
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Phosphorus, Sulfur, and Silicon, 186:1635–1649, 2011
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2011.582801
THE THERMAL AND MASS SPECTRAL
FRAGMENTATION OF N-BUTYL THIOLO-, THIONO-,
AND DITHIO-CHLOROFORMATE
Harry R. Hudson and Andrew J. Koplick
London Metropolitan University, London, UK
GRAPHICAL ABSTRACT
Abstract Thermal decomposition of n-butyl thiolochloroformate at 150◦C follows a similar
pattern to that reported previously for n-butyl chloroformate, to give butyl chloride which
is largely rearranged to the s-butyl isomer. An ion-pair mechanism, involving 1,2- and 1,3-
hydride shifts, is proposed. The less stable thiono- and dithio-chloroformates decompose to give
lower yields of butyl chlorides (mainly without rearrangement) together with numerous other
byproducts, indicating the operation of a more complex combination of reaction pathways.
The mass spectra of all three thio compounds exhibit molecular ions; the most prominent
fragment ions in their spectra are the n-butyl cation, and the radical cations COS+ • and
CS2+ •. Numerous chlorine-containing ions of low intensity are also observed, and their mode
of formation is discussed.
Keywords Carbocation rearrangement; dithiochloroformate; mass spectrometry; protonated
cyclopropane intermediate; thiochloroformate
INTRODUCTION
We showed in an earlier series of articles that the thermal decomposition of alkyl
chloroformates (1) in the liquid phase invariably gives rise to mixtures of alkyl halide
Received 19 March 2011; accepted 17 April 2011.
We thank Professor J. Charalambous for helpful discussions.
Address correspondence to Harry R. Hudson, London Metropolitan University, 166-220 Holloway Road,
London N7 8DB, UK. E-mail: harryrhudson@aol.com
1635

1636
H. R. HUDSON AND A. J. KOPLICK
isomers formed via the rearrangement of intermediate carbocations. The involvement of an
ion-pair intermediate was postulated (Scheme 1).1,2
Scheme 1
Straight-chain primary alkyl chloroformates such as the n-butyl (1, R = Bun) and n-pentyl
(1, R = n-C5H9) esters gave high proportions (ca. 80%) of rearranged secondary alkyl
chlorides (2-chlorobutane, or 2- and 3-chloropentane, respectively) resulting not only from
1,2-hydride shifts but also to a large extent by direct 1,3-hydride transfer via what may be
assumed to be protonated cyclopropane intermediates.3,4 Similarly, 1,3-methyl shifts were
implicated in the case of branched alkyl groups typified by neohexyl.4
In contrast with the chloroformates, straight-chain primary alkyl chlorosulfites (2),
e.g., n-butyl chlorosulfite (2, R = Bun), decompose to give no trace of rearranged
products—a result that can be ascribed to the easier liberation of halide ion by S(O) Cl
fission followed by SN2-type displacement at the α-carbon atom (Scheme 2).5
Scheme 2
In order to investigate the effect of replacing oxygen by sulfur in chloroformates, we have
now studied the thermal decomposition of n-butyl thiolo- (3), thiono- (4), and dithio- (5)
chloroformate, each of which was prepared by reported procedures and characterized by
NMR, IR, and mass spectrometry. The preparations and synthetic uses of thiocarbonic acid
derivatives, including thiochloroformates, have been reviewed elsewhere.6
RESULTS AND DISCUSSION
Our present investigations are concerned with the thermal decomposition of the thio-
and dithio-chloroformates, as compared to simple alkyl chloroformates, and the influence
of the replacement of oxygen by sulfur on the mechanism of alkyl halide formation. The

N-BUTYL THIOLO-, THIONO-, AND DITHIO-CHLOROFORMATE
1637
n-butyl esters (3, 4, and 5) were chosen as examples for study, and the formation of s-butyl
chloride was taken as evidence for the involvement of a carbocation intermediate. More
detailed work on 1,2- vs. 1,3-hydride shifts was not carried out, but it is likely that both are
involved as in the case of n-alkyl chloroformates.3,4 Mass spectral fragmentation patterns
for the three thio-compounds (3, 4, and 5), which do not appear to have been studied
previously in detail, are also reported.
Preparations
n-Butyl thiolochloroformate (3) was obtained by the interaction of butane-1-thiol with
an excess of liquid phosgene (Scheme 3), using a procedure similar to that described pre-
viously for alkyl chloroformates.2 Although the preparation of thiolochloroformates from
these reactants7,8 may involve the use of pyridine for the removal of hydrogen chloride,9 or a
variety of catalysts for promotion of the reaction, the avoidance of these additional reagents
simplifies the workup procedure, and we obtained satisfactory results without their use.
Analogous procedures for the preparation of n-butyl thionochloroformate (4) and n-butyl
dithiochloroformate (5) from butan-1-ol or butane-1-thiol and thiophosgene, although re-
ported in the literature,10–12 were less successful in our experience. We therefore prepared
these compounds by the interaction of sulfuryl chloride with bis(n-butoxythiocarbonyl)
disulfide (6) or bis(n-butylmercaptothiocarbonyl) disulfide (7), respectively (Scheme 4), as
described.13,14
Scheme 3
Scheme 4
The thionochloroformate (4) was the least stable of the three, and although it was
characterized immediately after preparation by NMR and mass spectrometry (including
exact mass measurement of the molecular ion), elemental analysis indicated contamination
with ca. 10% of inorganic material, which corresponded analytically to sulfur monochlo-
ride. n-Butyl thionochloroformate (4) has previously been reported to decompose during

1638
H. R. HUDSON AND A. J. KOPLICK
distillation,11 and sulfur monochloride has also been identified as a by-product in the prepa-
ration and distillation of the methyl and ethyl esters.15 For our present purpose of gaining
an insight into the mechanism of alkyl halide formation from the thionochloroformate (4),
we considered our 90% pure product to be suitable. Even if sulfur monochloride had been
rigorously removed, more would almost certainly have been generated under the conditions
of thermal decomposition.15
1H NMR data for compounds 3 and 4 agreed closely with those reported else-
where,16,17 with the α-CH2S and α-CH2O protons showing characteristic signals at δ 2.92
and 4.53, respectively. The signal for the α-CH2S protons of the dithio compound 5 (δ
3.18) was slightly downfield from that for compound 3, in accord with the lower electron-
attracting effect of C S compared to C O. Exact mass measurements for the molecular
ions of all three compounds confirmed their elemental composition. Further details of the
mass spectra are given below. The infrared spectra (see the Experimental section) showed
strong absorption maxima in the expected regions for C O, C S, C O, C S, and C Cl
stretching vibrations.18
Thermal Decomposition
Results for the thermal decomposition of n-butyl thiolo- (3), thiono- (4), and dithio-
(5) chloroformate in the liquid phase at 150◦C are summarized in Table 1; previous results
for n-butyl chloroformate (1, R = Bun) are included for comparison. Based on the amount
of unchanged starting material remaining after specified times of heating, the relative
stabilities are in the order:
Table 1 Thermal decomposition of n-butyl thiolo- (3), thiono- (4), and dithio- (5) chloroformates at 150◦C
Product composition (mol%)
Product(s)/mol. equiv.
RCl
Butene
c-2
Compound
Wt/g
Time/h
RCl
Butene
RXCYCl
BunCl
BusCl
1-
t-2
22
ROCOCla
7.8
1.4
1.2
10.8
8.4
41
0.16
0.17
0.14
0.50
0.13
0.30
0.06
0.21
0.01
0.00
0.54
0.76c
0.61e
0.00
0.00
22
40
39
98
99.8
78
60
61
2
67
96
99
70
0
11
4
1
30
0
3
250b
100
<2f
2–5g
3d
4
5
0.2
aData from Clinch and Hudson.2
bAnalysis every 50 h showed the ratio of butyl chloride isomers to remain essentially constant ( 1%) throughout.
cIn addition, a trace (0.005 mol) of (BuS)2CO was detected (glc).
dPyridine hydrochloride (3.1 mg, 0.4 mol%) was added.
e(BuS)2CO (0.025 mol) was detected (glc).
fAfter heating for a total of 100 h, a crystalline precipitate (0.91 g) was filtered off, recrystallized, and identified
as sulfur.
gAfter heating for 50 h, the distilled product (5.2 g) was subjected to prep. glc to give di-n-butyl disulfide
(0.31 g), di-n-butyl trithiocarbonate (2.0 g), and ca. 8 unidentified minor components.

N-BUTYL THIOLO-, THIONO-, AND DITHIO-CHLOROFORMATE
1639
Compounds containing the C(O) Cl grouping (3, 1) are significantly more stable than
those containing C(S) Cl (5, 4), due no doubt to the higher polarizability of the C Cl
bond in the latter and the easier liberation of the chloride ion; also, within these pairs of
compounds, the lower susceptibility to nucleophilic cleavage of R S compared to R O
can be correlated with the stability orders observed (3 > 1 and 5 > 4).
Product composition during the course of thermal decomposition was monitored for
the thiolochloroformate (3), and it was seen (Table 1) that the relative proportions of n-
and s-butyl chloride formed do not change significantly with time, as the composition of
the medium changes, suggesting the involvement of a common first intermediate for both
products. A similar observation was made previously for alkyl4 and cycloalkyl19 chloro-
formates. It is not necessary to invoke the formation of an unstable primary carbocation
in these processes; the evidence for a 1,3-H shift in the case of n-butyl chloroformate
(1, R = Bun)3,4 indicates the involvement of a protonated cyclopropane intermediate, in
which the positive center is stabilized through σ-delocalization,20 and a similar mechanism
(Scheme 5) is considered likely in the thermal decomposition of the thiolochloroformate
(3), leading to the formation of both n- and s-butyl chloride (routes a and b, respectively).
Rearrangement via a simple 1,2 H-shift (route c) cannot be excluded, although this was
shown to be of minor importance in the case of n-butyl chloroformate.4
Scheme 5
The decomposition of 3 occurred cleanly to give butyl halides and butenes as the
only significant organic products. Only a trace (ca. 0.5 mol%) of the disproportionation
product (BunS)2CO was detected. In this respect, the decomposition of 3 is similar to
that of n-butyl chloroformate (1, R = Bun), although rearrangement to s-butyl chloride
(60%) is rather less than in the case of 1 (R = Bun) (78%), a difference which might

1640
H. R. HUDSON AND A. J. KOPLICK
perhaps be due to the different environments and lifetimes of the ion-pair intermediates
in the two systems. A striking difference is, however, evident when the two compounds
are decomposed thermally in the presence of a catalytic quantity of pyridinium chloride.
In the case of n-butyl chloroformate (1, R = Bun), the formation of s-butyl chloride is
totally suppressed as a chain of SN2 displacements on carbon, similar to that shown for
the chlorosulfite (2) in Scheme 2, is initiated.1,2 However, the resistance of R S bonds to
nucleophilic cleavage inhibits this route in the decomposition of 3, and the ion-pair route
(with alkyl group rearrangement) is followed.
The formation of but-1-ene as the predominant elimination product (Table 1) suggests
(on the basis of Saytzeff’s rule) that this is not formed via a carbocation intermediate,
although it is worth noting that previous studies on deuterium-labeled n-butyl chloroformate
indicated that a significant proportion of but-1-ene could have been derived from the s-butyl
cation, in the particular circumstances of an ion-pair collapse.4 Alternatively, intramolecular
cis-elimination could be involved, similar to that suggested previously for chloroformates.2
Whether such a process is fully concerted (Scheme 6, route a) or, alternatively, C Cl
cleavage occurs in advance of deprotonation at Cβ, is a matter of detail, but in either case,
the terminal alkene will be formed. Evidence for bimolecular attack of chloride ion at Cβ in
the presence of pyridinium chloride (Scheme 6, route b) is provided by the observation of a
marked increase in the amount of but-1-ene relative to the amount of substitution products
formed under these conditions (Table 1).
Scheme 6
The thionochloroformate (4) and dithiochlorformate (5) are significantly less stable
than 3, and the butyl chloride formed in both cases consisted almost entirely of the unrear-
ranged n-butyl isomer. In both cases, the butyl chloride is therefore considered to be formed
mainly by SN2 displacement at Cα (by a similar sequence to that shown for the chlorosulfite
2 in Scheme 2). The overall reaction systems involved in the thermal decompositions of
these two compounds (4 and 5) are, however, complex. The thionochloroformate (4) de-
composed completely in under 2 h to give not only a 50% yield of alkyl halide (containing
only 2% of the rearranged s-butyl isomer) together with a small yield of butenes (ca. 1%),
but also at least 15 unidentified by-products as shown by gas-liquid chromatography (glc),
sulfur, and a black intractable residue. Complications may arise in this case by the unavoid-
able presence of sulfur monochloride (see above under the Preparations section), which may
give rise to additional side reactions.15 Thermal decomposition of the dithiochloroformate

N-BUTYL THIOLO-, THIONO-, AND DITHIO-CHLOROFORMATE
1641
(5) yielded a smaller amount (13%) of butyl chloride (99.8% n- and 0.2% s-isomer), and no
detectable butenes, with the main identifiable products being di-n-butyl trithiocarbonate (a
disproportionation product) and di-n-butyl disulfide. Again a black residue remained. The
overall complexity of the reaction products from both 4 and 5 suggests the involvement of
several mechanistic pathways, possibly both heterolytic and homolytic in character.
Mass Spectrometry
Mass spectral data for 3, 4, and 5 are shown in Tables 2–4. All three compounds
exhibit molecular ions, in contrast to n-butyl chloroformate (1, R = Bun), for which
the molecular ion is not detectable. The presence of sulfur evidently confers increased
stability (presumably through delocalization of charge as in the case of thiols, thioethers,
and thioeters21), and with two sulfur atoms present in n-butyl dithiochloroformate (5), the
molecular ion is the most intense (27% based on 35Cl).
Table 2 Ion abundances (m/z, %) in the mass spectra of n-butyl thiolo- (3), thiono- (4), and dithio- (5) chloro-
formates RXCYCl (R = n-Bu)
RSCOCl (3)
ROCSCl (4)
RSCSCl (5)
ROCOCl (1)
X = O, Y = O
Assignments
M+ •
X = S,
Y = O
X = O,
Y = S
X = S,
Y = S
154
152
125
123
124
122
119
117
111
109
98
1.6
4.4
0.6
2.0
1.0
11.0
1.5
20.0
0.4
1.0
0.5
0.8
7.0
2.0
6.0
154
152
2.5
6.5
170
168
141
139
124
122
135
133
12.0
27.0
1.1
2.9
0.5
3.0
15.0
29.0
—
—
—
—
—
—
•
M+ • – C2H5
—
—
M+ • – CH2O (S)a
124
122
119
117
0.8
1.5
0.6
1.6
M+ •– H3S• (or Cl•)b
—
—
—
—
—
—
•
M+ • – C3H7
ClCXYH+ •
—
—
—
—
98
96
89
81
79
65
63
61
60
57
56
0.5
0.9
1.0
6.0
14.0
5.0
13.0
3.0
71.0
78.0
100.0
114
112
89
81
79
21.0c
44.0c
5.0
18.0
41.0
96
89
65
63
C4H9S+
CYCl+
65
63
22.0
61.0
CXCl+
—
—
—
—
CXYH+
CXY++
C4H9+ •
C4H8
61
60
57
56
5.0
17.0
100.0
49.0
77
76
57
56
18.0
100.0
78.0
57
56
18.0
12.0
68.0
aThe presence of ions at m/z 124 and 122 indicates the loss of CH2O for compounds 3 and 4 and CH2S for
compound 5. However, the disproportionately intense ion at m/z 122 for compound 1 indicates the presence at the
same mass number of an additional unidentified species.
bThe two ions at m/z 119 and 117 indicate a chlorine-containing species formed by the loss of H3S, but the
relatively high intensity of the peak at m/z 117 for compound 3 suggests the presence of an additional ion at this
mass number, formed by the loss of Cl; the corresponding [M – Cl]+ ions derived from 4 and 5 (although less
prominent) were identified by exact mass measurement (see Results and Discussion section).
cIn the case of 5, additional speaks at m/z 113 (18%) and 115 (7.0%) correspond to the even-electron ion
+.
ClCXYH2

1642
H. R. HUDSON AND A. J. KOPLICK
Table 3 Exact mass measurements for selected ions∗
Compound
Nominal mass
Measured mass
Ion assignment
Calculated mass
n-BuSCOCl (3)
152
123
109
89
152
117
60
168
133
112
79
152.0063
122.9663
108.9523
89.0433
152.0069
117.0377
59.9672
167.9839
133.0141
111.9217
78.9412
C5H9OClS (M+)
C3H4OClS
C2H2OClS
C4H9S
152.0062
122.9670
108.9514
89.0424
152.0062
117.0374
59.9670
167.9834
133.0145
111.9208
78.9410
n-BuOCSCl (4)
n-BuSCSCl (5)
C5H9OClS (M+)
C5H9OS
COS
C5H9Cl2S2 (M+)
C5H9S2
CHClS2
CSCl
∗All based on 35Cl and 32S.
Table 4 Metastable ions in the mass spectra of n-butyl thiolo- (3), thiono- (4), and dithio- (5) chloroformates
m∗
Transition (R = n-Bu)
m/z for transition
Found
Calc.
Neutral fragment
RSCOCl (3)
[RSCOCl]+ • → [COCl]+
[RSCOCl]+ • → [RS]+
[RSCOCl]+ • → [RSCO]+
[RSCO]+ → R+
152 → 63
152 → 89a
152 → 117
117 → 57
117 → 60
96 → 60a
96 → 61
26.7
51.7
90.0
28.0
30.0
37.2
38.9
26.1
52.1
90.1
27.8
30.1
37.5
38.8
RS•
ClCO•
Cl•
COS
R•
[RSCO]+ → [COS]+ •
[ClCSOH]+ • → [COS]+ •
[ClCSOH]+ • → [CSOH]+
ROCSCl (4)
HCl
Cl•
[ROCSCl]+ • → [CSCl]+
[ROCSCl]+ • → [ROCS]+
[ROCSCl]+ • → [C3H7SCCl]+ •
[ROCS]+ → [COS]+ •
[ROCS]+ → R+
152 → 79
152 → 117
152 → 122
117 → 60
117 → 57
96 → 60
41.0
90.0
97.2
30.0
28.0
37.1
39.0
41.1
90.1
97.9
30.1
27.8
37.5
38.8
RO•
Cl•
CH2O
R•
COS
HCl
Cl•
[ClCOSH]+ • → [COS]+ •
[ClCOSH]+ • → [CSOH]+
RSCSCl (5)
96 → 61
[RSCSCl]+ • → [CSCl]+
[RSCSCl]+ • → [RSCS]+
[RSCSCl]+ • → [RS]+
[RSCSCl]+ • → [ClCSSH]+ •
[RSCSCl]+ • → [C3H7SCCl]+ •
[RSCSCl]+ • → [C3H4S2Cl]+
[RSCS]+ → R+
168 → 79
168 → 133
168 → 89
168 → 112
168 → 122
168 → 139a
133 → 57
133 → 76
112 → 76
112 → 77
37.0
105.0
47.0
74.0
89.0
115.0
25.0
43.0
51.1
53.0
37.1
105.2
47.2
74.6
88.6
115.0
24.7
43.6
51.5
52.9
RS•
Cl•
CSCl•
C4H8
CH2S
•
C2H5
CS2
R•
HCl
Cl•
[RSCS]+ → [CS2]+ •
[ClCSSH]+ • → [CS2]+ •
[ClCSSH]+ • → [CS2H]+
aWeak.

N-BUTYL THIOLO-, THIONO-, AND DITHIO-CHLOROFORMATE
1643
Apart from the molecular ion, the most prominent ions in the mass spectra of all three
+
n-butyl thiochloroformates include the butyl cation C4H9 (m/z 57), which gives rise to
the base peak in the spectrum of the thiolo derivative (3) and which occurs with relatively
high intensity (78%) for compounds 4 and 5. Whether the butyl cation is formed by
direct cleavage of the molecular ion is uncertain, but the evidence of metastable transitions
(Table 4) is in accord with its formation (Scheme 7, route a) via the stepwise loss of chlorine
and COS (from compounds 3 and 4), or chlorine and CS2 (from compound 5). (An ion at
m/z 56, corresponding to the radical cation C4H8+ • is also intense for all three compounds).
Scheme 7
By a parallel route, the loss of chlorine followed by the butyl radical (Scheme 6, route b)
gives prominent peaks due to the radical cations COS+ • (m/z 60) and CS2 (m/z 76).
+ •
In the case of compound 5, the ion at m/z 76 (CS2+ •) gives rise to the base peak (again
illustrating the stabilizing effect of two sulfur atoms). These same ions are also formed
by the elimination of HCl from ClCSOH+ or ClCS2H+, themselves formed by the loss of
butene from the molecular ions (Scheme 8), although it is ClCS2H+ only (containing two
sulfur atoms) that exhibits prominent peaks at m/z 112 (44%)/114 (21%). Analogous losses
from 3 and 4 give ions with relative intensities of < 1%.
Scheme 8
Other chlorine-containing ions, which are identified by the characteristic isotope pattern
for 35Cl/37Cl, are evident in the spectrum of all three thio compounds (3, 4, and 5) but are
mainly of very low intensity. These are exemplified among the higher mass numbers by the
loss of ethyl or propyl radicals from the molecular ion of 3 (Scheme 9), and by the loss of
CH2O or CH2S from 4 and 5, respectively (Scheme 10).
Elimination of ethyl or propyl radicals was negligible in the case of 4, and from 5,
the loss of ethyl only was observed.
The elimination of CH2O from 4 and of CH2S from 5 (Scheme 10) shows analogy
with the fragmentation of n-butyl acetate, as postulated by Black et al.,22 and is supported
by metastable transitions, although the corresponding ion (m/z 106), which might have
arisen from n-butyl chlorothioloformate (3), was absent.

1644
H. R. HUDSON AND A. J. KOPLICK
Scheme 9
Scheme 10
The formation of the ions m/z 122/124 in the spectrum of 3 appears to require prior
rearrangement to the thiono structure 4, followed by fragmentation as shown (Scheme 10).
However, the disproportionately high intensity of the peak at m/z 122 indicates the presence
of another (unidentified) species at this mass number.
The pair of chlorine-containing ions at m/z 117/119 corresponds to the loss from the
molecular ion of a neutral fragment of mass 35, for which the only reasonable assignment
is H3S. Although the H3S· radical has been reported elsewhere as a metastable species,23,24
it is difficult to envision a plausible mechanism by which such a fragment could be formed
in one step from the thio- or dithio-chloroformates, and a more complicated fragmentation
may be involved (e.g., the stepwise loss of H· and of H2S, or H2 and HS·). However, the
relatively high intensity of the peak at m/z 117 (in the case of compound 3) indicates the
presence of an additional species, [RXCY]+, at this same mass number, formed by the loss
of chlorine (as shown above in Scheme 7). The formation of this ion (and also analogous
ions at m/z 117 and 133 in the spectra of compounds 4 and 5, respectively, although of
much lower intensity) was supported by the observation of metastable transitions for all
three compounds, and in the case of compounds 4 and 5, by exact mass measurement
(Table 3).
The molecular ions of compounds 3 and 5 each give rise to the RS+ cation (m/z
89) by cleavage of the RS C bond (Scheme 11). The spectrum of 4, however, shows no
evidence for the formation of the analogous [RO]+ at m/z 73. The appearance of an ion at
m/z 89 in the spectrum of 4 appears to require prior rearrangement of R from O to S.
Other significant ions are formed directly from the molecular ions by:
(a) RX–C fission with the loss of the RX· radical to give CYCl+ (Scheme 12)

N-BUTYL THIOLO-, THIONO-, AND DITHIO-CHLOROFORMATE
1645
Scheme 11
Scheme 12
(b) the formation of [ClCXYH]+, most probably by a McLafferty rearrangement
(Scheme 13)
In the case of the dithiochloroformate (5), this ion was confirmed by exact mass
measurement, and the transition was metastable supported. It was a strikingly intense
ion (m/z 112, 44%), which can fragment further by either the loss of HCl or of Cl•
to give to ions [CXY]+ • or [CXYH]+, respectively. (All these secondary reactions
are metastable supported). The most stable ions from both routes are [CS2]+ • and
[CS2H]+ at m/z 76 (100%) and 77 (18%), in accord with the reported mass spectra
of sulfur compounds in general, in which sulfur atoms give added stability to positive
ions.21
Scheme 13
(c) A “Scho¨nberg-type” rearrangement25 (in the case of the thionoformate, 4), in which
the alkyl group migrates from oxygen to sulfur (Scheme 14). It is noteworthy that
this type of 1,3-migration from oxygen to sulfur does not occur readily under ther-
mal conditions in the case of simple alkyl groups. It was shown to occur to a lim-
ited extent in the case of methyl and ethyl thionochloroformate.15 Although we did
not observe 1,3-O to S migration during thermal decomposition of the n-butyl ester
(4), rearrangement clearly occurs in the molecular ion under the conditions of mass
spectrometry.

1646
H. R. HUDSON AND A. J. KOPLICK
Scheme 14
CONCLUSIONS
Using the n-butyl esters as typical examples for study, we have found that the thi-
olochloroformate (3) is thermally more stable than the corresponding chloroformate (1,
R = Bun), but that the thiono- (4) and dithio-compounds (5) are much less stable. These
observations have been rationalized on the basis of the relative electronegativities of oxy-
gen and sulfur and the susceptibilities of R O and R S bonds to nucleophilic cleavage.
Thermal decomposition of the thiolochloroformate (3) gives rise to a mixture of butyl chlo-
ride isomers in which the s-butyl isomer predominates. An ion-pair mechanism involving a
protonated cyclopropane intermediate is proposed. Butyl chlorides formed from thiono- (4)
and dithio-chloroformates (5) are largely unrearranged, but numerous other byproducts are
formed by what may be a complex combination of heterolytic and homolytic pathways. The
mass spectra of all thio- or dithio-chloroformates (3, 4, and 5) show the stabilizing effect
of sulfur on the molecular ion, which is not detectable in the case of n-butyl chloroformate
(1, R = Bun). The most prominent ions in the mass spectra of all three thio compounds are
the n-butyl cation and the radical cations COS+ • and CS2
.
+ •
EXPERIMENTAL
Starting Materials and Analytical Standards
Butan-1-ol, butane-1-thiol, phosgene, thiophosgene, and sulfuryl chloride were
obtained commercially and used as supplied. Described methods were used to pre-
pare potassium n-butyl xanthate26 and potassium n-butyl trithiocarbonate,27 and to
convert them by oxidation to bis(n-butoxythiocarbonyl) disulfide (6)28–30 and bis(n-
butylmercaptothiocarbonyl) disulfide (7),27 respectively. Di-n-butyl disulfide26 and
S,S’-di-n-butyl dithiocarbonate31 were prepared as described for use as gas-liquid
chromatography (glc) standards.
Analytical and Instrumental Methods
Microanalytical data (C, H) were obtained on a Perkin Elmer 240 instrument. Analysis
for chlorine and sulfur was carried out by Alfred Bernhardt Microanalytical Laboratory.
Mass spectra were run at 70 eV on an AEI MS-902 spectrometer at PCMU, Harwell, and 1H
NMR spectra on Perkin Elmer R10 or R12B instruments, operating at 60 MHz (ref TMS).
IR spectra were obtained for liquid films on Perkin Elmer 157 or Infracord spectrometers

N-BUTYL THIOLO-, THIONO-, AND DITHIO-CHLOROFORMATE
1647
with KBr plates. Analytical gas chromatography was carried out on a Perkin-Elmer F11
instrument with a flame ionization detector and nitrogen as carrier gas. Butyl halide isomers
and butenes were identified and determined using a squalane wall-coated stainless steel
capillary column (50 m × 0.25 mm i.d.), or a stainless steel column (2 m × 1/8 in o.d.)
containing 10% polyethylene glycol adipate on 85–100 mesh Celite. The latter was also
used to monitor the progress of the reaction and to determine unreacted starting materials
and other components. Preparative scale glc was carried out on a Pye series 105 instrument
with FID, stream splitter, nitrogen carrier gas, and a glass column (2.1 m × 9 mm o.d.)
containing 20% silicone oil (SE30) on 85–100 mesh Celite.
Preparations of Thio- and Dithio-chloroformates
n-Butyl thiolochloroformate (3). Butane-1-thiol (15.0 g, 0.166 mol) was dis-
solved in sodium-dried ether (50 cm3) and added dropwise under nitrogen (0.5 h) to freshly
distilled liquid phosgene (30.5 g, 0.308 mol) at −80◦C. The contents of the reactor were al-
lowed to warm to room temperature (3 h) and left stirring overnight, with hydrogen chloride
and excess phosgene being collected in traps containing 50% aqueous potassium hydrox-
ide. Final traces of phosgene were removed in a stream of nitrogen, ether was removed
under reduced pressure, and the residue was distilled to give n-butyl thiolochloroformate
(3) (14.0 g, 0.092 mol, 56%), bp 22–26◦C at 0.2 mmHg (lit.9,32 bp 179◦C at 760 mmHg;
lit.17 66–67◦C at 14 mmHg). (Found: C, 39.42; H, 5.92; Cl, 22.96; S, 21.27. Calc. for
C5H9ClOS: C, 39.34; H, 5.94; Cl, 23.23; S, 21.01%), nD 1.4783 (lit.9 nD 1.4736), δH
2.92 (t, JHCCH 6.7 Hz, 2H, CH2SCOCl), 0.7–2.0 (m, 7H, CH3CH2CH2), υmax/cm−1 1770
(C O), 840 (C S), 578 (C Cl); m/z (%) 152.0063 (4) (M+, calc. for C5H935ClO32S:
152.0062).
20
30
n-Butyl thionochloroformate (4)13. Sulfuryl chloride (6.7 g, 0.50 mol) was
added dropwise (30 min) to a stirred solution of bis(n-butoxythiocarbonyl) disulfide (14.9 g,
0.50 mol) in petroleum ether (bp 30–40◦C) (50 cm3) at room temperature. After a further 30
min, the mixture was heated under reflux (40–60◦C), with stirring. Atmospheric moisture
was excluded throughout by means of a calcium chloride drying tube. Distillation of the
product gave a mixture (7.4 g), bp 58–64◦C at 21 mmHg (lit.13 for 3, 58◦C at 21 mmHg; lit.16
74–79◦C at 30 mmHg), nD20 1.5004, of n-butyl thionochloroformate (4) (90%, 0.437 mol,
43.7% yield) and (by elemental analysis) sulfur monochloride (10%), [Found: C, 35.58;
H, 5.37; Cl, 27.36; S, 24.40. Calc. for C5H9ClOS (90%) + S2Cl2 (10%): C, 35.4; H,
5.35; Cl, 26.17; S, 23.65%], δH 4.53 (t, JHCCH 6.0 Hz, 2H, CH2OCSCl), 0.7-2.1 (m, 7H,
CH3CH2CH2), υmax/ cm−1 1023 (C O), 1274 (C S), 709 (C Cl); m/z (%) 152.0069 (7)
[M+, calc. for C5H935ClO32S (3): 152.0062].
n-Butyl dithiochloroformate (5)14. Sulfuryl chloride (3.5 g, 0.026 mol) was
added dropwise (30 min) to a stirred solution of bis(n-butylmercaptothiocarbonyl) disulfide
(7.2 g, 0.022 mol) in petroleum ether (bp 30–40◦C) (40 cm3) at room temperature. The
mixture was then heated under reflux (2 h), followed by fractional distillation through a
1 × 10 cm Vigreux column to give n-butyl dithiochloroformate (5) (4.1 g, 0.024 mol, 56%),
bp 108◦C at 21 mmHg (lit.14 92–95◦C at 10 mmHg) (Found: C, 35.58; H, 5.41; Cl, 21.01;
20
S, 38.01. Calc. for C5H9ClS2: C, 35.60; H, 5.38; Cl, 21.27; S, 38.12%), nD 1.5866, δH
3.18 (t, JHCCH 7.4 Hz, 2H, CH2SCSCl), 0.7–2.1 1 (m, 7H, CH3CH2CH2), υmax/cm−1 1112

1648
H. R. HUDSON AND A. J. KOPLICK
(C S), 787 (C S), 690 (C Cl); m/z (%) 167.9839 (27) (M+, calc for C5H935Cl32S2: 167.
9834).
Thermal Decompositions
n-Butyl thiolo- (3), thiono- (4), and dithio-(5) chloroformate were each heated under
reflux in an oil bath at 150◦C for the specified times (Table 1). Volatile products were
either retained in the system by a cold finger (−80◦C) or collected in a cold trap (−80◦C).
The total products from 1 were analyzed directly by glc, and those from 3 and 5 by
distillation followed by glc of the several fractions obtained. The product from 4 (10.8
g) was filtered to remove a crystalline precipitate (0.91 g), which was recrystallized from
carbon tetrachloride (25 cm3), and identified as sulfur (0.3 g), mp 113◦C (lit.33 115.21◦C,
monoclinic), m/z 256 (M+, S8). The filtrate was distilled through a Vigreux column (1 ×
10 cm) to give (i) 2.40 g, bp 79–80◦C at 760 mmHg, containing 1-chlorobutane (96%),
2-chlorobutane (2%), butenes (1%), and unidentified components (1%); (ii) 0.35 g, bp
75–76◦C at 12–13 mmHg, containing ca. 15 unidentified components; and (iii) a black
residue (0.51 g). The product from 5 (8.4 g) was distilled to give (i) 0.6 g, bp 70–79◦C
at 760 mmHg, containing 1-chlorobutane (99.5%), 2-chlorbutane (0.2%), and unidentified
components (0.3%); (ii) 5.2 g, bp 113–115◦C at 0.01 mmHg; (iii) a black residue (0.8 g).
Fraction (ii) (5.2 g) was subjected to prep. glc to give di-n-butyl disulfide (0.31 g, 90–95%
pure by glc, tR 10 min), m/z (%) 178 (M+, 33) and di-n-butyl trithiocarbonate (2.0 g, 99%
pure by glc, tR 22 min) (Found: C, 49.20; H, 7.73; S, 44.54. Calc. for C9H18S3: C, 48.59; H,
8.16; S, 43.25%), δH 3.36, (CH2S, t, JHCCH 7.0 Hz, 4H), 0.7–2.0 (CH2CH2CH2, m, 14H),
m/z(%) 222 (M+, 6), υmax/cm−1 1053 (C S), 827 (C S), and ca. 8 unidentified minor
components.
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